Process for producing polycarbonate

ABSTRACT

In a process for producing a polycarbonate by subjecting a dihydroxy compound and a diester of carbonic acid to melt polycondensation through transesterification, after a polycarbonate prepolymer is prepared in the first-stage reaction, polycondensation is conducted as the second-stage reaction at a residence time in the range of from 15 to less than 60 min or through the use of at least one paddle-type self-cleaning twin-screw extruder. 
     This process enables a colorless, transparent polycarbonate having a high molecular weight to be produced at a high efficiency on an industrial scale.

FIELD OF INDUSTRIAL APPLICATION

The present Invention relates to a process for producing apolycarbonate. More particularly, the present invention is concernedwith a process for producing a polycarbonate by subjecting a dihydroxycompound and a bisaryl carbonate to melt polycondensation throughtransesterification.

BACKGROUND ART

Polycarbonates are a general-purpose engineering thermoplastic and havefound a wide range of applications, particularly in injection molding ora glass sheet as an alternative to a windowpane.

Interfacial polycondensation, transesterification and other methods havehitherto been applied to the production of these polycarbonates.Although interfacial polycondensation is generally effective for theproduction of a polycarbonate, it has drawbacks such as the use of toxicphosgene as a starting compound for the reaction and the remainingchlorine ions generated during the reaction in the resultantpolycarbonate.

In order to eliminate the above-described drawbacks of the interfacialpolycondensation, Japanese Patent Publication-A No. 182336/1988discloses a process for producing a polycarbonate wherein a liquidtrichloromethyl chloroformate as a dimer of phosgene is used instead ofthe toxic phosgene and subjected to interfacial polycondensation with aspecial dihydric phenol. This document, however, cites only9,9-bis(4-hydroxyphenyl)fluorenes as the special dihydric phenol.

Angew. Chem., 99, 922 (1987) and German Patent DE 3440141 describe aprocess for producing a polycarbonate wherein triphosgene is usedinstead of the toxic phosgene and reacted with2,2-bis(4-hydroxyphenyl)propane. This process, however, involves also areaction mechanism whereby phosgene is generated.

In a process for producing a polycarbonate known in the art regardingthe transesterification, a high-molecular-weight polycarbonate isproduced by adding a transesterification catalyst to diphenyl carbonateand an aromatic dihydroxy compound as the starting compounds for thereaction, heating the mixture under reduced pressure to prepare aprepolymer while distilling off phenol and finally heating theprepolymer to 290° C. or above in a high vacuum to distill off phenol(see U.S. Pat. No. 4,345,062). However, it is known that, unlike otherengineering plastics, the melt viscosity of the high-molecular-weightpolycarbonate is so high that the reaction should be conducted at atemperature as high as 290° C. or above and that a high vacuum (10⁻²Torr) is necessary for distilling off phenol having a high boilingpoint, so that the industrialization of this process is difficult alsofrom the viewpoint of facilities, and the remaining phenol in theresultant polycarbonate has an adverse effect on the hue and propertiesof the polycarbonate.

However, various studies have been made on the transesterificationbecause it can be conducted through melt polycondensation and is atechnique which is excellent in profitability from the viewpoint ofindustry. In particular, since the viscosity of the reaction systembecomes high when the polycondensation approaches completion, attemptshave been made to use various types of apparatuses for the purpose oftreating a reaction product having a high viscosity (Japanese PatentPublication-B No. 36159/1977 and Japanese Patent Publication-A Nos.86618/1990 and 153923 to 153927/1990).

In the Japanese Patent Publication-B No. 36159/1977, the use of anintermeshing twin-screw extruder of a screw evaporator type isdisclosed. This extruder, however, has problems of forming a blackforeign matter in the residence section of screw grooves, the difficultyof controlling the residence time, the occurrence of coloring of theproduct due to the heating and the difficulty of efficiently removingdistillates such as phenol as a by-product.

In the Japanese Patent publication-A Nos. 153923 to 153927/1990, use ismade of a horizontal agitating polymerization tank. When this tank isused, although a large hold-up volume can be attained, the solutionthickness becomes so large that it becomes difficult to distill offphenol as the by-product from the reaction mixture having a highviscosity. This determines the rate of the reaction, and consequentlythe residence time should be prolonged. A long residence time means thatthe formed polycarbonate is exposed to a high temperature for a longperiod of time, which is causative of the coloring of the reactionproduct.

Thus, the conventional transesterification process is unsatisfactory forefficiently producing a colorless, transparent polycarbonate having ahigh molecular weight on an industrial scale.

DISCLOSURE OF THE INVENTION

The present invention eliminates the above-described drawbacks of theconventional process for producing a polycarbonate throughtransesterification and provides a process for producing a colorless,transparent polycarbonate having a high molecular weight at a highefficiency.

According to the first embodiment of the present invention, there isprovided a process for producing a polycarbonate by subjecting adihydroxy compound and a diester of carbonic acid to meltpolycondensation by transesterification, characterized in that after apolycarbonate prepolymer is prepared in the first-stage reaction,polycondensation is conducted as the second-stage reaction at aresidence time in the range of from 15 to less than 60 min.

According to the second embodiment of the present invention, there isprovided a process for producing a polycarbonate by subjecting adihydroxy compound and a diester of carbonic acid to meltpolycondensation by transesterification, characterized in that after apolycarbonate prepolymer is prepared in the first-stage reaction,polycondensation is conducted as the second-stage reaction through theuse of at least one paddle-type self-cleaning twin-screw extruder.

The present invention will now be described in more detail.

At the outset, the starting compounds for the reaction (monomers) willbe described.

In the present invention, a wide variety of compounds may be used as thedihydroxy compound, and examples thereof includebis(hydroxyaryl)alkanes, bis(hydroxyaryl)arenes,bis(hydroxyaryl)cycloalkanes, dihydroxydiaryl ethers, dihydroxydiarylsulfides, dihydroxydiaryl sulfoxides and dihydroxydiaryl sulfones.

Among them, preferred dihydroxy compounds are compounds represented bythe following general formulae (I), (II) and (III): ##STR1## (whereinR₁, R₂, R₃, R₄, R₅ and R₆ which may be the same or different, each standfor a hydrogen atom, an alkyl group, a cycloalkyl group, an aralkylgroup or an aryl group, X₁, X₂, Y₁, Y₂, Y₃, Z₁ and Z₂, which may be thesame or different, each stand for a halogen atom, an alkoxy group, analkyl group, a cycloalkyl group, an aralkyl group or an aryl group, l,m, n, p, q, r and s are each an integer of 0 to 4 and t is an integer of2 to 10).

Examples of the alkyl group include straight-chain or branched alkylgroups such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl,tert-butyl, n-pentyl, isopentyl, neopentyl, hexyl, heptyl, octyl,2-ethylhexyl, decyl, undecyl and dodecyl groups. The alkyl group ispreferably a straight-chain or branched alkyl group having about 1 to 8carbon atoms.

Examples of the cycloalkyl group include cyclopropyl, cyclopentyl,cyclohexyl, cycloheptyl and cyclooctyl groups.

Examples of the aralkyl group include benzyl, phenethyl and benzhydrylgroups. Examples of the aryl group inclule phenyl, naphthyl and anthrylgroups. The aryl group is preferably a phenyl group.

Examples of the halogen atom include fluorine, chlorine, bromine andiodine atoms. Examples of the alkoxy group include methoxy, ethoxy,n-propoxy, isopropoxy, n-butoxy, isobutoxy, pentyloxy and hexyloxygroups.

The above-described alkyl, cycloalkyl, aryl and aralkyl groups may haveat least one substituent which is inert under reaction conditions.Examples of such a substituent include a halogen atom, an alkoxy group,an aryloxy group, an aralkyloxy group, an alkylthio group, a cyano groupand a nitro group.

The dihydroxy compound is preferably a compound represented by thegeneral formulae (I), (II) or (III) wherein the substituents R₁ to R₆each stand for a hydrogen atom, an alkyl group (particularly an alkylgroup having 1 to 8 carbon atoms) or an aryl group (particularly aphenyl group) and the substituents X₁, X₂, Y₁, Y₂, Y₃, Z₁ and Z₂ eachstand for an alkyl group (particularly an alkyl group having 1 to 8carbon atoms), an aryl group (particularly a phenyl group) or a halogenatom.

Compounds represented by the following general formulae (IV), (V), (VI)and (VII) as well are preferred as the dihydroxy compound: ##STR2##(wherein R₇, R₈, R₉ and R₁₀ each stand for a hydrogen atom, astraight-chain or branched alkyl group having 1 to 8 carbon atoms or aphenyl group, X₃ stands for a halogen atom, u is 0 to 4 and w is 1 to4).

Examples of the dihydroxy compound to be used in the present inventioninclude 2,2-bis-(4-hydroxy-phenyl)propane,2,2-bis(4-hydroxyphenyl)butane,2,2-bis(4-hydroxyphenyl)-4-methylpentane,2,2-bis(4-hydroxyphenyl)octane, 4,4'-dihydroxy-2,2,2-triphenylethane,2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane,2,2-bis(4-hydroxy-3-methylphenyl)propane,2,2-bis(4-hydroxy-3-isopropylphenyl)propane,2,2-bis(4-hydroxy-3-sec.butylphenyl)propane,2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane,2.2-bis(4-hydroxy-3-tert-butylphenyl)propane,1,1'-bis(4-hydroxyphenyl)-p-diisopropylbenzene,1,1'-bis(4-hydroxyphenyl)-m-diisopropylbenzene and1,1-bis(4-hydroxyphenyl)cyclohexane. It is also possible to produce acopolycarbonate through the use of a combination of two or at leastthree of the above-described dihydroxy compounds.

Examples of the diester of carbonic acid to be used in the presentinvention include bisaryl carbonates such as diphenyl carbonate,bis(2,4-dichlorophenyl) carbonate, bis(2,4,6-dichlorophenyl) carbonate,bis(2-cyanophenyl) carbonate, bis(o-nitrophenyl) carbonate, ditolylcarbonate, m-cresyl carbonate, dinaphthyl carbonate and bis(diphenyl)carbonate; dicycloalkyl carbonates such as dicyclohexyl carbonate; anddialkyl carbonates such as dimethyl carbonate and diethyl carbonate.Among these diesters of carbonic acid, bisaryl carbonates are preferred,and diphenyl carbonate is particularly preferably used.

Regarding the proportions of the starting compounds for the reaction,the diester of carbonic acid is generally used in an amount of 0.90 to1.50 mol, preferably 0.95 to 1.25 mol, still preferably about 1.00 to1.01 mol based on one mole of the dihydroxy compound.

In the present invention, if necessary, use may be made of apolymerization catalyst, and when a polymerization catalyst is used, itmay be any conventional transesterification catalyst.

Representative examples of the trans-esterification catalyst useable inthe present invention include (a) catalysts belonging to ametal-containing catalyst, such as lithium borohydride, sodiumborohydride, potassium borohydride, rubidium borohydride, cesiumborohydride, beryllium borohydride, magnesium borohydride, calciumborohydride, strontium borohydride, barium borohydride, aluminumborohydride, titanium borohydride, tin borohydride, germaniumborohydride, tetraphenoxylithium, tetraphenoxysodium,tetraphenoxypotassium, tetraphenoxyrubidium, tetraphenoxycesium, sodiumthiosulfate, beryllium oxide, magnesium oxide, tin (IV) oxide,dibutyltin oxide, beryllium hydroxide, magnesium hydroxide, germaniumhydroxide, beryllium acetate, magnesium acetate, tin (IV) acetate,germanium acetate, lithium carbonate, sodium carbonate, potassiumcarbonate, beryllium carbonate, magnesium carbonate, tin (IV) carbonate,germanium carbonate, tin (IV) nitrate, germanium nitrate, antimonytrioxide and bismuth trimethylcarboxylate.

As (b) catalysts belonging to an electron-donating amine compound,N,N-dimethyl-4-aminopyridine, 4-diethylaminopyridine, 4-aminopyridine,2-aminopyridine, 2-hydroxypyridine, 2-methoxypyridine,4-methoxypyridine, 4-hydroxypyridine, 2-dimethylaminoimidazole,2-methoxyimidazole, 2-mercaptoimidazole, aminoquinoline, imidazole,2-methylimidazole, 4-methylimidazole, diazabicyclooctane (DABCO) and thelike are cited.

Further, the examples of the catalyst include (c) carbonate, acetate,formate, nitrate, nitrite, oxalate, fluoroborate, hydrofluoride, etc.,of the above electron-donating amine compound.

As (d) catalysts belonging to an electron-donating phosphorus compound,triethylphosphine, tri-n-propylphosphine, triisopropylphosphine,tri-n-butylphosphine, triphenylphosphine,tri-o-dimethoxyphenylphosphine, tri-p-tolylphosphine,tri-o-tolylphosphine, tributyl phosphlte, triphenyl phosphite,tri-p-tolyl phosphite, tri-o-tolyl phosphite and the like are cited.

Furthermore, (e) catalysts belonging to a borane complex includecomplexes comprising borane and a following compound, that is, ammonia,dimethylamine, trimethylamine, triethylamine, tert-butylamine,dimethylaniline, pyridine, dimethylaminopyridine, morpholine,piperazine, pyrrole, tetrahydrofuran, dimethyl sulfide,tri-n-butylphosphine, triphenylphosphine, triphenyl phosphite or thelike.

These catalysts may be used alone or in a combination of two or more ofthem. The amount of use thereof may be usually in the range of from1×10⁻⁶ to 1 mol, preferably 5×10⁻⁵ to 5×10⁻² mol based on one mole ofthe dihydroxy compound. When the amount of use of the catalyst is lessthan 5×10⁻⁵ mol, the rate of polymerization becomes low, which oftengives rise to coloring of the reaction product. When the amount exceeds5×10⁻² mol, the catalyst remains in the resultant polycarbonate whichoften gives an adverse effect on the properties, for example, a loweringin the mechanical properties.

The catalyst may be fed at once in an early stage of the first-stagereaction. Alternatively, proper catalysts may be used respectively inthe first-stage reaction and the second-stage reaction.

The present invention is directed to a process for producing apolycarbonate through the transesterification of the above-describedstarting compounds for the reaction. In the present invention, use ismade of a two-stage reaction system. Specifically, the process of thepresent invention comprises the first-stage reaction wherein theabove-described starting compounds are transesterified with each otherin a molten state, optionally in the presence of a transesterificationcatalyst, to give a polycarbonate prepolymer and the second-stagereaction wherein the polycondensation is further allowed to proceedunder a highly viscous condition to give a polycarbonate having a highmolecular weight.

In the present invention, there is no particular limitation on thereaction conditions for the first-stage reaction. However, the reactiontemperature is usually in the range of from 100° to 300° C., preferablyin the range of from 130° to 280° C. When the reaction temperature isbelow 130° C., the rate of reaction becomes low, while when the reactiontemperature exceeds 280° C., side reactions are liable to occur. It ispreferred to conduct the reaction in an inert atmosphere.

In the first-stage reaction, phenol or the like is produced as aby-product accompanying the advance of the reaction. Such a by-productmay be distilled off under reduced pressure.

In the present invention, although there is no particular limitation onthe method of conducting the first-stage reaction, it is preferred toconduct the first-stage reaction through any one of the followingmethods (a) to (i).

(a) A method wherein the starting compounds for the reaction aresubjected to polycondensation in a molten state in at least onevessel-type reactor while distilling off phenol or other compoundsproduced in the reaction.

(b) A method which comprises the first step of subjecting the startingcompounds for the reaction to polycondensation in a molten state in atleast one vessel-type reactor to give a polycarbonate prepolymer havinga viscosity-average molecular weight of 5,000 to 20,000 while distillingoff phenol or other compounds produced in the reaction, the second stepof placing the prepolymer produced in the first step in at least onehold tank, and the third step of introducing the prepolymer produced inthe second step through at least one preheater into a vacuum flashchamber, where the prepolymer is subjected to polycondensation whiledistilling off phenol or other compounds produced in the reaction togive a polycarbonate prepolymer having a viscosity-average molecularweight of 10,000 to 30,000.

(c) A method which comprises the first step of subjecting the startingcompounds for the reaction to polycondensation in a molten state in atleast one vessel-type reactor to give a polycarbonate prepolymer havinga viscosity-average molecular weight of 5,000 to 20,000 while distillingoff phenol or other compounds produced in the reaction and the secondstep of introducing the prepolymer produced in the first step through atleast one preheater into a vacuum flash chamber, where the prepolymer issubjected to polycondensation while distilling off phenol or othercompounds produced in the reaction to give a polycarbonate prepolymerhaving a viscosity-average molecular weight of 10,000 to 30,000.

(d) A method which comprises the first step of melting the startingcompounds for the reaction in a vessel for melting starting compounds,the second step of subjecting the starting compounds for the reaction topolycondensation in a horizontal twin-shaft polymerizer to give apolycarbonate prepolymer having a viscosity-average molecular weight of5,000 to 30,000 while distilling off phenol or other compounds producedin the reaction, and the third step of placing the prepolymer producedin the second step in at least one hold tank.

(e) A method which comprises the first step of melting the startingcompounds for the reaction in a vessel for melting starting compoundsand the second step of subjecting the starting compounds for thereaction to polycondensation in a horizontal twin-shaft polymerizer togive a polycarbonate prepolymer having a viscosity-average molecularweight of 5,000 to 30,000 while distilling off phenol or other compoundsproduced in the reaction.

(f) A method which comprises the first step of subjecting the startingcompounds for the reaction to polycondensation in a molten state in atleast one vessel-type reactor to give a polycarbonate prepolymer havinga viscosity-average molecular weight of 5,000 to 20,000 while distillingoff phenol or other compounds produced in the reaction, the second stepof placing the prepolymer produced in the first step in at least onehold tank, and the third step of subjecting the prepolymer topolycondensation in at least one vented extruder while distilling offphenol or other compounds produced in the reaction to give apolycarbonate prepolymer having a viscosity-average molecular weight of10,000 to 30,000.

(g) A method which comprises the first step of subjecting the startingcompounds for the reaction to polycondensation in a molten state in atleast one vessel-type reactor to give a polycarbonate prepolymer havinga viscosity-average molecular weight of 5,000 to 20,000 while distillingoff phenol or other compounds produced in the reaction and the secondstep of subjecting the prepolymer to polycondensation in at least onevented extruder while distilling off phenol or other compounds producedin the reaction to give a polycarbonate prepolymer having aviscosity-average molecular weight of 10.000 to 30,000.

(h) A method which comprises the first step of melting the startingcompounds for the reaction in a vessel for melting the startingcompounds and the second step of subjecting the starting compounds forthe reaction to polycondensation in at least one vessel-type reactor togive a polycarbonate prepolymer having a viscosity-average molecularweight of 5,000 to 30,000 while distilling off phenol or other compoundsproduced in the reaction.

(i) A method which comprises the first step of melting the startingcompounds for the reaction in a vessel for melting the startingcompounds, the second step of subjecting the starting compounds for thereaction to polycondensation in at least one vessel-type reactor to givea polycarbonate prepolymer having a viscosity-average molecular weightof 5,000 to 20,000 while distilling off phenol or other compoundsproduced in the reaction and the third step of allowing thepolycondensation of the prepolymer produced in the second step tofurther proceed in at least one vessel-type reactor or horizontalpolymerizer while distilling off phenol or other compound produced inthe reaction to give a polycarbonate prepolymer having aviscosity-average molecular weight of 10,000 to 30,000.

The above-described methods (a) to (i) will now be described in moredetail.

(a) A method wherein the starting compounds for the reaction aresubjected to polycondensation in a molten state in at least onevessel-type reactor while distilling off phenol or other compoundproduced in the reaction:

The method (a) may be conducted either batchwise or continuously. Whenthe reaction is conducted batchwise, the vessel-type reactor to be usedmay be a usual one equipped with an agitator. The reaction may beconducted through the use of either one or a plurality of such reactors.Examples of the agitator include those equipped with turbine blades,paddle blades, anchor blades, helical ribbon blades and modified typesof the above-described blades. The agitator is particularly preferablyone equipped with helical ribbon blades or an improved type of thehelical ribbon blade which can be used in a high-viscosity vessel fromthe viewpoint of the molecular weight distribution and the shortening ofthe time for the reaction product to be held at a high temperature.

In the latter stage of the reaction, the viscosity of the reactionproduct becomes so high that the use of an agitator for a high viscosityequipped with blades such as paddle blades, grid blades or helicalblades is also favorable.

The vessel-type reactor to be used herein is preferably one providedwith a distillation column. The distillation column is useful foravoiding the escape of the starting compounds from the system togetherwith by-products such as phenol during their removal.

In the method (a), after a dihydroxy compound and a diester of carbonicacid as the starting compounds for the reaction are melted in an inertatmosphere, preferably a nitrogen atmosphere, in the above-describedvessel-type reactor, a transesterification catalyst is added thereto anda reaction is allowed to proceed while agitating. With the advance ofthe reaction, the temperature is gradually raised and the pressure isgradually reduced to distill off phenol produced as a by-product.

In order to compensate for the loss of the quantity of heat due to thelatent heat of vaporization, which occurs with the elimination of aby-product such as phenol, a draft tube or the like is provided with thereaction vessel for the purpose of supplying the heat.

In practicing the method (a), the reaction temperature and the degree ofvacuum are preferably in the range of from 150° to 300° C. and in therange of from 400 to 1 Torr, respectively.

The viscosity-average molecular weight of the polycarbonate prepolymerproduced by the method (a) is usually 5,000 to 20,000.

(b) A method which comprises the first step of subjecting the startingcompounds for the reaction to polycondensation in a molten state in atleast one vessel-type reactor to give a polycarbonate prepolymer havinga viscosity-average molecular weight of 5,000 of 20,000 while distillingoff phenol or other compounds produced in the reaction, the second stepof placing the prepolymer produced in the first step in at least onehold tank, and the third step of introducing the prepolymer produced inthe second step through at least one preheater into a vacuum flashchamber, where the prepolymer is subjected to polycondensation whiledistilling off phenol or other compounds produced in the reaction togive a polycarbonate prepolymer having a viscosity-average molecularweight of 10,000 to 30,000:

The first step of this method corresponds to the method (a).

In the second step, the prepolymer produced in the first step is placedin a hold tank.

In the hold tank, the prepolymer is maintained in a molten state andoptionally agitated by means of an agitator to homogenize theprepolymer. In this case, it is possible to supply additives, such as ahue improver, an antioxidant and a catalyst terminal protecting agent,in such an amount as will have no adverse effect on the reaction in thesubsequent step, and this method is included in the scope of the presentinvention.

In the third step, the prepolymer from the second step is introducedthrough at least one preheater into a vacuum flash chamber, wherepolycondensation is allowed to proceed while distilling off phenol orother compounds produced in the reaction.

Examples of the preheater include multi-tubular, double-pipe, platy,static mixer and other heat exchangers. In the third step, theprepolymer is heated through the use of these preheaters and fed into avacuum flash chamber, where polycondensation is allowed to proceed whileevaporating by-products such as phenol under reduced pressure.

In the vacuum flash chamber, the reaction temperature and the degree ofvacuum are usually 200° to 320° C. and 50 to 0.5 Torr, respectively.

The viscosity-average molecular weight of the polycarbonate prepolymerproduced by the method (b) is usually 10,000 to 30,000.

(c) A method which comprises the first step of subjecting the startingcompounds for the reaction to polycondensation in a molten state in atleast one vessel-type reactor to give a polycarbonate prepolymer havinga viscosity-average molecular weight of 5,000 to 20,000 while distillingoff phenol or other compounds produced in the reaction and the secondstep of introducing the prepolymer produced in the first step through atleast one preheater into a vacuum flash chamber, where the prepolymer issubjected to polycondensation while distilling off phenol or othercompounds produced in the reaction to give a polycarbonate prepolymerhaving a viscosity-average molecular weight of 10,000 to 30,000:

The method (c) is the same as the method (b), except that the secondstep is omitted. Specifically, in the method (c), the prepolymerproduced in the first step is directly introduced through at least onepreheater into a vacuum chamber, where the polycondensation is furtherallowed to proceed while distilling off the by-product, therebypreparing a polycarbonate prepolymer having a viscosity-averagemolecular weight of 10,000 to 30,000.

(d) A method which comprises the first step of melting the startingcompounds for the reaction in a vessel for melting starting compounds,the second step of subjecting the starting compounds for the reaction topolycondensation in a horizontal twin-shaft polymerizer to give apolycarbonate prepolymer having a viscosity-average molecular weight of5,000 to 30,000 while distilling off phenol or other compounds producedin the reaction, and the third step of placing the prepolymer producedin the second step in at least one hold tank:

The vessel for melting the starting compounds to be used in the firststep may be a conventional agitation vessel. In the first step, thestarting compounds for the reaction may be melted while agitating at aproper temperature selected according to the starting compounds for thereaction.

In the second step, a polycarbonate prepolymer is produced through theuse of a horizontal twin-shaft polymerizer.

The second step may be conducted either batchwise or continuously.Further, the reaction may be conducted through the use of a plurality ofhorizontal twin-shaft polymerizers.

The term "horizontal twin-shaft polymerizer" used herein is intended tomean a polymerizer having two horizontal shafts of rotation equippedwith disk, pin, spectacle, wheel or other type of blades.

When use is made of a plurality of horizontal twin-shaft polymerizers,in the latter stage of the reaction, it is preferred to use apolymerizer of a high hold type among the horizontal polymerizerswherein the residence time is one hour or more.

The horizontal twin-shaft polymerizer to be used herein is preferablyone provided with a distillation column. The distillation column isuseful for avoiding the escape of the starting compounds from the systemtogether with by-products such as phenol during their removal.

In the second step, a dihydroxy compound and a diester of carbonic acidas the starting compounds for the reaction are reacted with each otherin a molten state in an inert atmosphere, preferably in a nitrogenatmosphere, in the presence of a transesterification catalyst in theabove-described horizontal twin-shaft polymerizer. When the second stepis conducted batchwise, the temperature is gradually raised and thepressure is gradually reduced with the advance of the reaction todistill off phenols produced as the by-product.

Further, in order to compensate for the loss of quantity of heat due tothe latent heat of vaporization which occurs with the elimination of aby-product such as phenol, a draft tube or the like is provided with thereaction vessel for the purpose of supplying the heat.

In the second step, the reaction temperature and the degree of vacuumare preferably 150° to 300° C. and 400 to 1 Torr, respectively. Theviscosity-average molecular weight of the polycarbonate prepolymerproduced in the second step is usually 5,000 to 20,000.

In the third step, the prepolymer produced in the second step is placedin a hold tank. This step is the same as the second step of the method(b).

The viscosity-average molecular weight of the polycarbonate prepolymerproduced by the method (d) is usually 10,000 to 30,000.

(e) A method which comprises the first step of melting the startingcompounds for the reaction in a vessel for melting starting compoundsand the second step of subjecting the starting compounds for thereaction to polycondensation in a horizontal twin-shaft polymerizer togive a polycarbonate prepolymer having a viscosity-average molecularweight of 5,000 to 30,000 while distilling off phenol or other compoundsproduced in the reaction: The method (e) is the same as the method (d),except that the third step is omitted, that is, comprises the first andsecond steps of the method (d).

The viscosity-average molecular weight of the polycarbonate prepolymerproduced by the method (e) is usually 10,000 to 30,000.

(f) A method which comprises the first step of subjecting the startingcompounds for the reaction to polycondensation in a molten state in atleast one vessel-type reactor to give a polycarbonate prepolymer havinga viscosity-average molecular weight of 5,000 to 20,000 while distillingoff phenols or other compound produced in the reaction, the second stepof placing the prepolymer produced in the first step in at least onehold tank, and the third step of subjecting the prepolymer topolycondensation in at least one vented extruder while distilling offphenol or other compounds produced in the reaction to give apolycarbonate prepolymer having a viscosity-average molecular weight of10,000 to 30,000:

The first and second steps of this method are the same as the first andsecond steps of the method (b), respectively.

In the third step, the prepolymer produced in the second step isintroduced into at least one vented extruder, where the polycondensationis allowed to proceed while distilling off phenol or other compoundsproduced in the reaction.

The vented extruder may be a single-screw or twin-screw extruder, andmonovented, two-vented and other extruders are known in the art. In thisstep, use may be made of a conventional single-screw monovented extruderwhich is advantageous from the viewpoint of the cost.

In the vented extruder, the reaction temperature and the degree ofvacuum are usually 200° to 320° C. and 50 to 0.5 Torr, respectively.

The viscosity-average molecular weight of the polycarbonate prepolymerproduced by the method (f) is usually 10,000 to 30,000.

(g) A method which comprises the first step of subjecting the startingcompounds for the reaction to polycondensation in a molten state in atleast one vessel-type reactor to give a polycarbonate prepolymer havinga viscosity-average molecular weight of 5,000 of 20,000 while distillingoff phenol or other compounds produced in the reaction and the secondstep of subjecting the prepolymer to polycondensation in at least onevented extruder while distilling off phenol or other compounds producedin the reaction to give a polycarbonate prepolymer having aviscosity-average molecular weight of 10,000 to 30,000:

The method (g) is the same as that of the method (f), except that thesecond step is omitted. Specifically, in the method (g), the prepolymerproduced in the first step is directly introduced into at least onevented extruder, where the polycondensation is further allowed toproceed while distilling off the by-products, thereby preparing apolycarbonate prepolymer having a viscosity-average molecular weight of10,000 to 30,000.

(h) A method which comprises the first step of melting the startingcompounds for the reaction in a vessel for melting the startingcompounds and the second step of subjecting the starting compounds forthe reaction to polycondensation in at least one vessel-type reactor togive a polycarbonate prepolymer having a viscosity-average molecularweight of 5,000 to 30,000 while distilling off phenol or other compoundsproduced in the reaction:

The first step of this method is the same as the first step of themethod (d).

The second step corresponds to the method (a). In this case, however, itis preferred to conduct the reaction in the second step of the method(h) under the conditions of a temperature in the range of from 130° to300° C. and a pressure in the range of from 400 to 0.5 Torr.

The viscosity-average molecular weight of the polycarbonate prepolymerproduced by the method (h) is usually 5,000 to 30,000.

(i) A method which comprises the first step of melting the startingcompounds for the reaction in a vessel for melting the startingcompounds, the second stop of subjecting the starting compounds for thereaction to polycondensation in at least one vessel-type reactor to givea polycarbonate prepolymer having a viscosity-average molecular weightof 5,000 to 20,000 while distilling off phenol or other compoundsproduced in the reaction and the third step of allowing thepolycondensation of the prepolymer produced in the second step tofurther proceed in at least one vessel-type reactor and/or at least onehorizontal polymerizer while distilling off phenol or other compoundsproduced in the reaction to give a polycarbonate prepolymer having aviscosity-average molecular weight of 10,000 to 30,000:

The first and second steps of this method are the same as the first andsecond steps of the method (h), except that in the second step, thereaction is conducted under such a condition that the resultantpolycarbonate prepolymer has a viscosity-average molecular weight of5,000 to 20,000.

In the third step, use is made of a vessel-type reactor and/or ahorizontal polymerizer, and the polycondensation of the polycarbonateprepolymer produced in the second step is further allowed to proceed. Inthis step, the removal of phenol or other compounds formed in thereaction is also conducted.

When the third step is conducted in a vessel-type reactor,fundamentally, the procedure of the second step may be repeated. In thiscase, however, since the polycarbonate as the star-ting compound has aviscosity-average molecular weight of 5,000 to 20,000, it is favorableto use a vessel-type reactor provided with an agitator for a highviscosity having paddle blades, grid blades, helical blades or otherblades.

On the other hand, when the third step is conducted in a horizontalpolymerizer, use is made of a high hold type horizontal polymerizerwhich has one or two horizontal shafts of rotation equipped with disk,pin, spectacle, wheel or other blades and is generally used at aresidence time of one hour or more.

The third step may be conducted either batchwise or continuously.Further, it is also possible to conduct the reaction through the use ofa plurality of vessel-type reactors and horizontal polymerizers.

The vessel-type reactor and horizontal polymerizer are preferably onesprovided with a distillation column. The distillation column is usefulfor avoiding the escape of the starting compounds from the systemtogether with by-products such as phenol during their removal.

In order to compensate for the loss of the quantity of heat due to thelatent heat of vaporization which occurs with the elimination of aby-product such as phenol, a draft tube or the like may be provided withthe vessel-type reactor and horizontal polymerizer for the purpose ofsupplying the heat.

In the third step, the reaction temperature and the degree of vacuum arepreferably in the range of from 200° to 320° C. and 10 to 0.5 Torr,respectively.

The viscosity-average molecular weight of the polycarbonate prepolymerproduced by the method (i) is usually 10,000 to 30,000.

According to the process of the present invention, after a polycarbonateprepolymer is produced in the above-described first-stage reaction, thesecond-stage reaction is conducted to give a polycarbonate having a highmolecular weight.

In the step of the post-condensation (second-stage reaction), as thepolycondensation approaches the completion, the viscosity of thereaction system becomes so high that it is necessary to conduct thetreatment at a high temperature. Further, in order to increase themolecular weight, it was very important to find a method which enablesside reactions (formation of a coloring source) due to oxidativedeterioration to be prevented while distilling off by-products, etc.

As a result of extensive studies, the present inventors have found thata high-molecular weight polycarbonate free from coloring can be obtainedby conducting the polycondensation at a residence time in the range offrom 15 to less than 60 min at the step of post-condensation(second-stage reaction) and conducting the step of post-condensation ata solution thickness in the range of from 0.1 to 50 mm (the firstembodiment of the present invention).

The term "residence time" used herein is intended to mean a period oftime which contributes to the polycondensation in a high vacuum at ahigh temperature. When the residence time is less than 15 min, theremoval of by-products becomes unsatisfactory, so that it is impossibleto increase the molecular weight. On the other hand, when the residencetime is 60 min or more, there is a high possibility that coloring mayproceed due to side reactions, etc., unfavorably.

The solution thickness is preferably 0.1 to 50 mm from the viewpoint ofefficiently removing by-products. When the solution thickness is lessthan 0.1 mm, it is difficult to attain a satisfactory profitability onan industrial scale. On the other hand, when the solution thicknessexceeds 50 mm, it becomes difficult to remove by-products. In an actualapparatus, the present invention includes an embodiment wherein thesolution thickness in the reaction section is locally 0.1 to 50 mm.

In the second-stage reaction in the process according to the firstembodiment of the present invention, the reaction temperature ispreferably in the range of from 180° to 350° C., still preferably in therange of from 200° to 320° C. When the temperature is below 200° C., itis often difficult to conduct the removal of phenol and otherby-products necessary for increasing the molecular weight. On the otherhand, when the temperature exceeds 320° C., the coloring of the resinand the side reactions are accelerated unfavorably.

The reaction is preferably conducted in a high vacuum, and the degree ofvacuum is preferably 10 Torr or less, still preferably 1 Torr or less.

There is no particular limitation on the apparatus to be used in thesecond-stage reaction in the process according to the first embodimentof the present invention so far as it can be used in this application.Examples of the apparatus include high-viscosity treatment apparatusessuch as a paddle-type self-cleaning twin-screw extruder, a (forced)thin-film evaporator and a horizontal twin-shaft polymerizer, amongwhich the paddle-type self-cleaning twin-screw extruder is preferred.

In contrast, in the second embodiment of the present invention, theconventional problems of the occurrence of black scorched foreign matterand the failure in producing a polymer having a sufficiently highmolecular weight can be solved by examining the second-stage reactionfrom the viewpoint of the apparatus.

Namely, in the process according to the second embodiment of the presentinvention, the above-described problems can be solved through conductingpolycondensation reaction with at least one paddle-type self-cleaningtwin-screw extruder in the step of post-condensation (second-stagereaction).

The term "paddle" used herein is intended to mean a discontinuous blade(that is, a blade whose bottom is not continuous with the bottom of anadjacent blade) as opposed to a screw. Although examples of the forminclude a convex lens and a pseudo-triangle, the form of the paddle isnot limited to those only.

By the term "self-cleaning" is meant a function brought about when apair of paddles built in two shafts, each in one shaft, have phasesshifted from each other in such a manner that they rotate while keepinga constant minute gap between them so that the head of one paddle cleansthe side of the other paddle.

In the self-cleaning twin-screw extruder to be used in the presentinvention, the L/D value (wherein L represents the length of the shaftand D represents the diameter of rotation of the paddle) is usually inthe range of from 1 to 35, preferably 1 to 20. In the present invention,the L/D value is not always limited to the above range.

In order to promote the kneading effect, use may be made of a twin-screwextruder having such a blade structure that a plurality of blades can berotated so as to be different from each other in the number ofrevolutions and have a self-cleaning function. In this case, theextruder is operated under such a condition that the blades aredifferent from each other in the number of revolutions, that is, undersuch a condition that the number of revolutions of the blade provided inone shaft are different from the number of revolutions of the bladeprovided in the other shaft.

Further, in the paddle-type self-cleaning twin-screw extruder to be usedin the present invention, the shafts may be provided horizontallyparallel to each other or vertically parallel to each other.

The use of the paddle-type self-cleaning twin-screw extruder enables theconditions of the renewal of the surface and the solution thickness tobe set so that phenol and other by-products produced in the reaction canbe efficiently removed. Further, a colorless, transparent polycarbonatehaving a high molecular weight can be produced through the control ofthe residence time.

The second-stage reaction in the process according to the secondembodiment of the present invention is usually conducted under theconditions of a reaction temperature of 220° to 350° C. and a degree ofvacuum of 5 to 0.1 Torr. The residence time is 0.2 to 2 hr, preferably0.4 to 1.2 hr, and the operation is continuously conducted. Accompanyingthe renewal of the surface of the reaction mixture, phenol and otherby-products are removed through a distillate port connected to a vacuumsystem. The viscosity-average molecular weight of the polycarbonateproduced by the process according to the second embodiment of thepresent invention is 12,000 to 60,000.

EFFECT OF THE INVENTION

According to the present invention, a colorless, transparentpolycarbonate having a high molecular weight can be produced at a highefficiency.

EXAMPLES

The present invention will now be described in more detail withreference to the following Examples, though it is not limited to theseExamples only.

Example 1

After a 20-l vessel-type agitator was charged with 4566 g (20.0 mol) ofbisphenol A and 4392 g (20.5 mol) of diphenyl carbonate as the startingcompounds for the reaction and 0.489 g (0.004 mol) of4-dimethylaminopyridine and 0.039 g (0.0004 mol) of potassium acetateand the air in the reaction system was purged with nitrogen, thetemperature was raised to 180° C. with agitating to melt the mixture.The reaction system was then evacuated to 2 Torr while gradually raisingthe temperature to 260° C. to react the starting compounds for thereaction with each other and, at the same time, to distill off phenol asa by-product. About 4 hr after the initiation of distilling off thephenol, a polycarbonate prepolymer having a viscosity-average molecularweight of 13,000 was obtained. Then, the polycarbonate prepolymer wassupplied into a paddle-type self-cleaning twin-screw extruder (with L/Dof 8.9, rotational diameter of paddle of 50 mm, and shaft length of445.5 mm) at 280° C. and 0.2 Torr and discharged at a rate of 1030 g/hrby means of a gear pump so that the residence time was 35 min. Theresultant polymer had a viscosity-average molecular weight of 28,000 anda hue (A₃₈₀ -A₅₈₀) of 0.08. The clearance (solution thickness) of thetwin-screw extruder was about 0.5 mm.

The viscosity-average molecular weight (Mn) was determined by measuringthe intrinsic viscosity [η] of the methylene chloride solution of thepolymer at 20° C. with an Ubbelohde viscometer and calculating theviscosity-average molecular weight according to the following equation:

    [η]=1.11×10.sup.-4 (Mv).sup.0.82

For the evaluation of the hue, a 10% methylene chloride solution ofpolycarbonate was prepared to measure the absorbances of the solution ata wavelength of 380 nm and 580 nm. The hue was expressed in thedifference in the absorbance between 380 nm and 580 nm. The larger thevalue, the higher the intensity of coloring.

Example 2

After a 20-l vessel-type agitator was charged with 4566 g (20.0 mol) ofbisphenol A and 4392 g (20.5 mol) of diphenyl carbonate as the startingcompounds for the reaction and 0.489 g (0.004 mol) of4-dimethylaminopyridine and 0.078 g (0.0008 mol) of potassium acetateand the air in the reaction system was purged with nitrogen, thetemperature was raised to 180° C. with agitating to melt the mixture.The reaction system was then evacuated to 2 Torr while gradually raisingthe temperature to 260° C. to react the starting compounds with eachother and, at the same time, to distill off phenol as a by-product.About 4 hr after the initiation of distilling off the phenol, apolycarbonate prepolymer having a viscosity-average molecular weight of16,000 was obtained. Then, the polycarbonate prepolymer was suppliedinto a paddle-type self-cleaning twin-screw extruder at 280° C. and 0.2Torr and discharged at a rate of 655 g/hr by means of a gear pump sothat the residence time was 55 min. The resultant polymer had aviscosity-average molecular weight of 34,000 and a hue (A₃₈₀ -A₅₈₀) of0.09.

Comparative Example 1

A polycarbonate prepolymer prepared in the same manner as that ofExample 1 was supplied into a horizontal twin-shaft polymerizer at 280°C. and 0.2 Torr and discharged by means of a gear pump so that theresidence time was 120 min. The resultant polymer had aviscosity-average molecular weight of 18,000 and a hue (A₃₈₀ -A₅₈₀) of0.65.

Comparative Example 2

A polycarbonate prepolymer prepared in the same manner as that ofExample 1 was supplied into a screw evaporator at 280° C. and 0.2 Torrand discharged by means of a gear pump so that the residence time was 12min. The resultant polymer had a viscosity-average molecular weight of16,000 and a hue (A₃₈₀ -A₅₈₀) of 0.45.

Comparative Example 3

A polycarbonate prepolymer prepared in the same manner as that ofExample 1 was supplied into a twin screw extruder at 280° C. and 0.2Torr and discharged by means of a gear pump so that the residence timewas 180 min. The resultant polymer had a viscosity-average molecularweight of 35,000. However, the hue (A₃₈₀ -A₅₈₀) was 1.20, and apartially blackened scorch was observed.

Example 3

After a 20-l vessel-type agitator was charged with 4566 g (20.0 mol) ofbisphenol A and 4392 g (20.5 mol) of diphenyl carbonate as the startingcompounds for the reaction and 0.489 g (0.004 mol) of4-dimethylaminopyridine and 0.039 g (0.0004 mol) of potassium acetateand the air in the reaction system was purged with nitrogen, thetemperature was raised to 180° C. with agitating to melt the mixture.The reaction system was then evacuated to 2 Torr while gradually raisingthe temperature to 260° C. to react the starting compounds with eachother and, at the same time, to distil off phenol as a by-product. About4 hr after the initiation of distilling off the phenol, a polycarbonateprepolymer having a viscosity-average molecular weight of 13,000 wasobtained. Then, the polycarbonate prepolymer was supplied into apaddle-type self-cleaning twin-screw extruder (with L/D of 8.9,rotational diameter of paddle of 50 mm, and shaft length of 445.5 mm) at280 ° C. and 0.2 Torr and discharged at a rate of 800 g/hr by means of agear pump. The residence time was about 45 min. The resultant polymerhad a viscosity-average molecular weight of 30,000 and a hue (A₃₈₀-A₅₈₀) of 0.09.

Example 4

After a 20-l vessel-type agitator was charged with 4566 g (20.0 mol) ofbisphenol A and 4392 g (20.5 mol) of diphenyl carbonate as the startingcompounds for the reaction and 0.328 g (0.004 mol) of 2-methylimidazoleand 0.033 g (0.0004 mol) of sodium acetate and the air in the reactionsystem was purged with nitrogen, the temperature was raised to 180° C.with agitating to melt the mixture. The reaction system was thenevacuated to 2 Torr while gradually raising the temperature to 260° C.to react the starting compounds with each other and, at the same time,to distil off phenol as a by-product. About 4.5 hr after the initiationof distilling off the phenol, a polycarbonate prepolymer having aviscosity-average molecular weight of 15,000 was obtained. Then, thepolycarbonate prepolymer was supplied into a paddle-type self-cleaningtwin-screw extruder (with L/D of 8.9, rotational diameter of paddle of50 mm, and shaft length of 445.5 mm) at 280 ° C. and 0.2 Torr anddischarged at a rate of 900 g/hr by means of a gear pump. The residencetime was about 40 min. The resultant polymer had a viscosity-averagemolecular weight of 32,000 and a hue (A₃₈₀ -A₅₈₀) of 0.10.

Example 5

After a 20-l vessel-type agitator was charged with 4566 g (20.0 mol) ofbisphenol A and 4392 g (20.5 mol) of diphenyl carbonate as the startingcompounds for the reaction and 0.489 g (0.004 mol) of4-dimethylamimopyridine and 0.039 g (0.0004 mol) of potassium acetateand the air in the reaction system was purged with nitrogen, thetemperature was raised to 180° C. with agitating to melt the mixture.The reaction system was then evacuated to 2 Torr while gradually raisingthe temperature to 260° C. to react the starting compounds with eachother and, at the same time, to distil off phenol as a by-product. About4 hr after the initiation of distilling off the phenol, a polycarbonateprepolymer having a viscosity-average molecular weight of 13,000 wasobtained. Then, the polycarbonate prepolymer was supplied into apaddle-type self-cleaning twin-screw extruder (with L/D of 8.9,rotational diameter of paddle of 50 mm, and shaft length of 445.5 mm)kept at 280° C. and 0.2 Torr and rotated in a ratio of the number ofrevolutions of blades of 2:1, and discharged at a rate of 800 g/hr bymeans of a gear pump. The residence time was about 55 min. The resultantpolymer had a viscosity-average molecular weight of 31,000 and a hue(A₃₈₀ -A₅₈₀) of 0.11.

Comparative Example 4

The procedure of Example 3 was repeated, except that a horizontaltwin-screw extruder was used instead of the paddle-type self-cleaningtwin-screw extruder, the extruder was maintained under the conditions ofa temperature of 280° C., a pressure of 0.2 Torr and the discharge wasconducted at a rare of 800 g/hr by means of a gear pump. The residencetine was about 1.5 hr. The resultant polymer had a viscosity-averagemolecular weight of 19,000 and a hue (A₃₈₀ -A₅₈₀) of 0.42.

Comparative Example 5

The procedure of Example 3 was repeated, except that a screw evaporatorwas used instead of the paddle-type self-cleaning twin-screw extruder,the evaporator was maintained under the conditions of a temperature of290° C., a pressure of 0.2 Torr and the discharge was conducted at arate of 850 g/hr by means of a gear pump. The residence time was about10 min. The resultant polymer had a viscosity-average molecular weightof 20,000 and a hue (A₃₈₀ -A₅₈₀) of 0.65, and a black scorch was locallyobserved.

Example 6

After a 20-l vessel-type agitator was charged with 4566 g (20.0 mol) ofbisphenol A and 4392 g (20.5 mol) of diphenyl carbonate as the startingcompounds for the reaction and 3.28 g (0.04 mol) of 2-methylimidazoleand 0.033 g (0.0004 mol) of sodium acetate and the air in the reactionsystem was purged with nitrogen, the temperature was raised to 180° C.with agitating to melt the mixture. The reaction system was thenevacuated to 2 Torr while gradually raising the temperature to 260° C.to react the starting compounds with each other and, at the same time,to distill off phenol as a by-product. About 4 hr after the initiationof distilling off the phenol, a polycarbonate prepolymer having aviscosity-average molecular weight of 13,000 was obtained. Then, thepolycarbonate prepolymer was transfered to a 12-l hold tank by means ofa gear pump and maintained at a temperature of 260° C. in a nitrogenatmosphere. The prepolymer contained in the hold tank was then suppliedinto a preheater by means of a gear pump, heated to 280° C. and fed intoa vacuum flash chamber at a temperature of 280° C. and a pressure of 1Torr, where the polycondensation was allowed to proceed. Theviscosity-average molecular weight of the polycarbonate prepolymer atthe time of the completion of this step was 20,000. Then, thepolycarbonate prepolymer was supplied into a paddle-type self-cleaningtwin-screw extruder (with L/D of 8.9, rotational diameter of paddle of50 mm, and shaft length of 445.5 mm) at 280° C. and 0.2 Torr anddischarged at a rate of 800 g/hr by means of a gear pump. The residencetime was about 45 min. The resultant polymer had a viscosity-averagemolecular weight of 30,000 and a hue (A₃₈₀ -A₅₈₀) of 0.101.

Example 7

A polycarbonate prepolymer having a viscosity-average molecular weightof 11,000 was prepared under the same charging condition as that ofExample 6 through the use of the same vessel-type agitator as that ofExample 6, except that 4.88 g (0.04 mol) of 4-dimethylaminopyrldine wasused as the catalyst instead of the 2-methylimidazole. The prepolymerwas fed by means of a gear pump into a vessel-type agitator of the typeas described above, where the polycondensation was allowed to furtherproceed, thereby preparing a polycarbonate prepolymer having aviscosity-average molecular weight of 15,000. The prepolymer was fedinto a preheater by means of a gear pump, heated to 280° C., fed into avacuum flash chamber at a temperature of 280° C. and a pressure of 1Torr, where the polycondensation was allowed to proceed. Theviscosity-average molecular weight of the polycarbonate prepolymer atthe time of the completion of this step was 22,000. Then, thepolycarbonate prepolymer was supplied into a paddle-type self-cleaningtwin-screw extruder at 280° C. and 0.2 Torr and discharged at a rate of900 g/hr by means of a gear pump. The resultant polymer had aviscosity-average molecular weight of 29,000 and a hue (A₃₈₀ -A₅₈₀) of0.105.

Comparative Example 6

The procedure of Example 6 was repeated, except that a horizontaltwin-screw extruder was used instead of the paddle-type self-cleaningtwin-screw extruder, the extruder was maintained under the conditions ofa temperature of 280° C., a pressure of 0.2 Torr and the discharge wasconducted at a rate of 800 g/hr by means of a gear pump. The residencetine was about 3 hr. The resultant polymer had a viscosity-averagemolecular weight of 22,000 and a hue (A₃₈₀ -A₅₈₀) of 0.27.

Comparative Example 7

The procedure of Example 6 was repeated, except that a screw evaporatorwas used instead of the paddle-type self-cleaning twin-screw extruder,the evaporator was maintained under the conditions of a temperature of280° C., a pressure of 0.2 Torr and the discharge was conducted at arate of 900 g/hr by means of a gear pump. The residence time was about10 min. The resultant polymer had a viscosity-average molecular weightof 24,000 and a hue (A₃₈₀ -A₅₈₀) of 0.50, and a black scorch was locallyobserved.

Example 8

4566 g (20.0 mol) of bisphenol A and 4392 g (20.5 mol) of diphenylcarbonate were melted in a vessel for melting staring compounds, and fedinto a 20-l horizontal twin-shaft polymerizer. The air present insidethe horizontal twin-shaft polymerizer was purged with nitrogen, and 3.28g (0.04 mol) of 2-methylimidazole and 0.033 g (0.0004 mol) of sodiumacetate were added as a catalyst at 180° C. with agitating. The reactionsystem was evacuated to 2 Torr while gradually raising the temperatureto 260° C. to react the starting compounds with each other and, at thesame time, to distill off phenol as a by-product. About 4 hr after theinitiation of distilling off the phenol, a polycarbonate prepolymerhaving a viscosity-average molecular weight of 16,000 was obtained.Then, the polycarbonate prepolymer was transferred to a 12-l hold tankby means of a gear pump and maintained at a temperature of 260° C. in anitrogen atmosphere. The prepolymer contained in the hold tank was thenfed, by means of a gear pump, into a paddle-type self-cleaningtwin-screw extruder (with L/D of 8.9, rotational diameter of paddle of50 mm, and shaft length of 445.5 mm) at a pressure of 0.2 Torr anddischarged at a rate of 800 g/hr by means of a gear pump. The residencetime was about 45 min. The resultant polymer had a viscosity-averagemolecular weight of 30,000 and a hue (A₃₈₀ -A₅₈₀) of 0.100.

Example 9

A 20-l horizontal twin-shaft polymerizer was charged with the startingcompounds in the same manner as that of Example 8, except that 4.88 g(0.04 mol) of 4-dimethylaminopyridine was used as the catalyst insteadof the 2-methylimidazole. After the air present inside the horizontaltwin-shaft polymerizer was purged with nitrogen, the reaction system wasevacuated to 4 Torr with agitating while gradually raising thetemperature from 180° to 260° C. to react the starting compounds witheach other and, at the same time, to distill off phenol as a by-product.About 4 hr after the initiation of distilling off the phenol, apolycarbonate prepolymer having a viscosity-average molecular weight of13,000 was obtained. Then, the polycarbonate prepolymer was fed into ahorizontal twin-shaft polymerizer of the type as described above, wherethe polycondensation was allowed to proceed under the conditions of atemperature of 280° C. and a pressure of 2 Torr. The resultantprepolymer had a viscosity-average molecular weight of 19,000. Theprepolymer was then fed into a paddle-type self-cleaning twin-screwextruder at a temperature of 280° C. and a pressure of 0.2 Torr anddischarged at a rate of 800 g/hr by means of a gear pump. The residencetime was about 45 min. The resultant polymer had a viscosity-averagemolecular weight of 32,000 and a hue (A₃₈₀ -A₅₈₀) of 0.090.

Comparative Example 8

The procedure of Example 8 was substantially repeated, except that ahorizontal twin-shaft polymerizer was used instead of the paddle-typeself-cleaning twin-screw extruder, the polymerizer was maintained underthe conditions of a temperature of 280° C., a pressure of 0.2 Torr andthe discharge was conducted at a rate of 800 g/hr by means of a gearpump. The residence time was about 3 hr. The resultant polymer had aviscosity-average molecular weight of 25,000 and a hue (A₃₈₀ -A₅₈₀) of0.17.

Comparative Example 9

The procedure of Example 8 was substantially repeated, except that ascrew evaporator was used instead of the paddle-type self-cleaningtwin-screw extruder, the evaporator was maintained under the conditionsof a temperature of 280° C., a pressure of 0.2 Torr and the dischargewas conducted at a rate of 900 g/hr by means of a gear pump. Theresidence time was about 10 min. The resultant polymer had aviscosity-average molecular weight of 24,000 and a hue (A₃₈₀ -A₅₈₀) of0.20, and a black scorch was locally observed.

Example 10

After a 20-l vessel-type agitator was charged with 4566 g (20.0 mol) ofbisphenol A and 4392 g (20.5 mol) of diphenyl carbonate as the startingcompounds for the reaction and 3.28 g (0.04 mol) of 2-methylimidazoleand 0.033 g (0.0004 mol) of sodium acetate and the air in the reactionsystem was purged with nitrogen, the temperature was raised to 180° C.with agitating to melt the mixture. The reaction system was thenevacuated to 2 Torr while gradually raising the temperature to 260° C.to react the starting compounds with each other and, at the same time,to distill off phenol as a by-product. About 4 hr after the initiationof distilling off the phenol, a polycarbonate prepolymer having aviscosity-average molecular weight of 13,000 was obtained. Then, thepolycarbonate prepolymer was transferred to a 12-l hold tank by means ofa gear pump and maintained at a temperature of 260° C. in a nitrogenatmosphere. The prepolymer contained in the hold tank was then suppliedinto a vented extruder by means of a gear pump, and the polycondensationwas allowed to proceed under the conditions of a temperature of 280° C.and a pressure of 1 Torr. The viscosity-average molecular weight of thepolycarbonate prepolymer at the time of the completion of this step was19,000. Then, the prepolymer was supplied, by means of a gear pump, intoa paddle-type self-cleaning twin-screw extruder (with L/D of 8.9,rotational diameter of paddle of 50 mm, and shaft length of 445.5 mm) at280° C. and 0.2 Torr and discharged at a rate of 800 g/hr by means of agear pump. The residence time was about 45 min. The resultant polymerhad a viscosity-average molecular weight of 29,000 and a hue (A₃₈₀-A₅₈₀) of 0.105.

Example 11

A polycarbonate prepolymer having a viscosity-average molecular weightof 11,000 was prepared under the same charging condition as that of theExample 10 through the use of the same vessel-type agitator as that ofthe Example 10, except that 4.88 g (0.04 mol) of 4-dimethylaminopyrldinewas used as the catalyst instead of the 2-methylimidazole. Theprepolymer was fed, by means of a gear pump, into a vessel-type agitatorof the type as described above, where the polycondensation was furtherallowed to proceed, thereby preparing a polycarbonate prepolymer havinga viscosity-average molecular weight of 15,000. The prepolymer was thenfed into a vented extruder by means of a gear pump, and thepolycondensation was allowed to proceed under the conditions of atemperature of 280° C. and a pressure of 1 Torr. The viscosity-averagemolecular weight of the polycarbonate prepolymer at the time of thecompletion of this step was 20,000. Then, the prepolymer was supplied,by means of a gear pump, into a paddle-type self-cleaning twin-screwextruder at 280° C. and 0.2 Torr and discharged at a rate of 900 g/hr bymeans of a gear pump. The resultant polymer had a viscosity-averagemolecular weight of 28,000.

Comparative Example 10

The procedure of Example 10 was substantially repeated, except that ahorizontal twin-shaft polymerizer was used instead of the paddle-typeself-cleaning twin-screw extruder, the polymerizer was maintained underthe conditions of a temperature of 280° C., a pressure of 0.2 Torr andthe discharge was conducted at a rate of 800 g/hr by means of a gearpump. The residence time was about 3 hr. The resultant polymer had aviscosity-average molecular weight of 23,000 and a hue (A₃₈₀ -A₅₈₀) of0.38.

Comparative Example 11

The procedure of Example 10 was substantially repeated, except that ascrew evaporator was used instead of the paddle-type self-cleaningtwin-screw extruder, the evaporator was maintained under the conditionsof a temperature of 280° C., a pressure of 0.2 Torr and the dischargewas conducted at a rate of 900 g/hr by means of a gear pump. Theresidence time was about 10 min. The resultant polymer had aviscosity-average molecular weight of 23,000 and a hue (A₃₈₀ -A₅₈₀) of0.55, and a black scorch was locally observed.

Example 12

After a 20-l vessel-type agitator was charged with 4566 g (20 mol) ofbisphenol A and 4392 g (20.5 mol) of diphenyl carbonate as the startingcompounds and 4.88 g (0.04 mol) of 4-dimethylaminopyridine and 0.033 g(0.0004 mol) of sodium acetate as catalysts and the air in the reactionsystem was purged with nitrogen, the temperature was raised to 180° C.with agitating to melt the mixture. Then, the mixture of the startingcompounds and the catalysts was fed into a 20-l vessel-type reactorequipped with a distillation column, and the vessel was evacuated to 2Torr while gradually raising the temperature from 180° to 260° C. toreact the starting compounds with each other and, at the same time, todistill off phenol as a by-product. About 4 hr after the initiation ofdistilling off the phenol, a polycarbonate prepolymer having aviscosity-average molecular weight of 13,000 was obtained. Then, thepolycarbonate prepolymer was fed, by means of a gear pump, into apaddle-type self-cleaning twin-screw extruder (with L/D of 8.9,rotational diameter of paddle of 50 mm, and shaft length of 445.5 mm) at280° C. and 0.2 Torr and discharged at a rate of 850 g/hr by means of agear pump. The residence time was about 45 min. The viscosity-averagemolecular weight (Mv) of the resultant polycarbonate was measured andfound to be 28,000. The hue (A₃₈₀ -A₅₈₀) of the polycarbonate was 0.110.

Example 13

After a polycarbonate prepolymer having a viscosity-average molecularweight of 13,000 was prepared in the same manner as that of Example 12,the polycarbonate prepolymer was transferred to a 20-l vessel-typereactor by means of a gear pump, where the polycondensation was allowedto further proceed under the conditions of a temperature of 280° C. anda pressure of 1 Torr. The viscosity-average molecular weight of thepolycarbonate prepolymer at the time of the completion of this step was20,000. Then, the prepolymer was supplied, by means of a gear pump, intoa paddle-type self-cleaning twin-screw extruder (with L/D of 8.9,rotational diameter of paddle of 50 mm, and shaft length of 445.5 mm) at280° C. and 0.2 Torr and discharged at a rate of 800 g/hr by means of agear pump. The residence time was about 45 min. The viscosity-averagemolecular weight (Mv) of the resultant polycarbonate was measured andfound to be 27,000. The hue (A₃₈₀ -A₅₈₀) of the polycarbonate was 0.101.

Example 14

A polycarbonate prepolymer having a viscosity-average molecular weightof 14,000 was prepared under the same charging conditions as those ofExample 12 except for the use of 3.28 g (0.04 mol) of 2-methylimidazoleas the catalyst instead of the 4-dimethylaminopyridine through the useof a vessel-type agitator and a vessel-type reactor provided with adistillation column of the same type as those used in Example 12. Theprepolymer was transferred, by means of a gear pump, to a vessel-typereactor of the type as described above, where the polycondensation wasallowed to further proceed to give a polycarbonate prepolymer having aviscosity-average molecular weight of 18,000. The prepolymer was thenfed, by means of a gear pump, into a paddle-type self-cleaningtwin-screw extruder at a temperature of 280° C. and a pressure of 0.2Torr and discharged at a rate of 900 g/hr by means of a gear pump. Theresultant polymer had a viscosity-average molecular weight of 29,000 anda hue (A₃₈₀ -A₅₈₀) of 0.105.

Example 15

The procedure up to the reaction in the vessel-type reactor wasconducted under the same charging and reaction conditions as those ofExample 12 to prepare a polycarbonate prepolymer having aviscosity-average molecular weight of 14,000. This prepolymer was fed,by means of a gear pump, into a horizontal polymerizer, where a reactionwas allowed to proceed under the conditions of a temperature of 280° C.and a pressure of 1 Torr to give a polycarbonate prepolymer having aviscosity-average molecular weight of 20,000, which was then fed, bymeans of a gear pump, into a paddle-type self-cleaning twin-screwextruder at a temperature of 280° C. and a pressure of 0.2 Torr anddischarged at a rate of 900 g/hr. The viscosity-average molecular weightof the resultant polymer was 30,000. The hue (A₃₈₀ -A₅₈₀) was 0.102.

Comparative Example 12

The first-stage reaction was conducted in the same manner as that ofExample 13 to give a polycarbonate prepolymer, which was then fed into ahorizontal twin-shaft polymerizer instead of the paddle-typeself-cleaning twin-screw extruder and discharged at a rate of 800 g/hrby means of a gear pump under the conditions of a temperature of 280° C.and a pressure of 0.2 Torr. The residence time was about 3 hr. Theviscosity-average molecular weight of the resultant polymer was 22,000.The hue (A₃₈₀ -A₅₈₀) was 0.27.

Comparative Example 13

The first-stage reaction was conducted in the same manner as that ofExample 13 to give a polycarbonate prepolymer, which was then fed into ascrew evaporator instead of the paddle-type self-cleaning twin-screwextruder and discharged under the conditions of a temperature of 300° C.and a pressure of 0.2 Torr at a rate of 900 g/hr by means of a gearpump. The residence time was about 10 min. The viscosity-averagemolecular weight of the resultant polymer was 24,000. The hue (A₃₈₀-A₅₈₀) was 0.50, and a black scorch was locally observed.

We claim:
 1. A process for producing a polycarbonate comprising (1) afirst stage reaction comprising melt polycondensing an aromaticdihydroxy compound and a diester of carbonic acid at a temperature from100° to 300° C. to form a polycarbonate prepolymer and, after theprepolymer has formed, (2) a second stage reaction comprisingpolycondensing the polycarbonate prepolymer for a time of from 15 toless than 60 minutes at a solution, thickness of 0.1 to 50 mm in ahorizontal twin-shaft polymerizer to form a polycarbonate having agreater viscosity-average molecular weight than the polycarbonateprepolymer formed in the first stage reaction.
 2. A process forproducing a polycarbonate comprising (1) a first stage reactioncomprising melt polycondensing an aromatic dihydroxy compound and adiester of carbonic acid at a temperature of from 100° to 300° C. toform a polycarbonate prepolymer having a viscosity-average molecularweight of from 5,000 to 20,000 and, after the prepolymer has formed, (2)a second stage reaction comprising polycondensing the polycarbonateprepolymer for a time of from 15 to less than 60 minutes, a temperatureof from 180° to 350° C. and at a solution thickness of 0.1 to 50 mm in ahorizontal twin-shaft polymerizer to form a polycarbonate having ahigher viscosity-average molecular weight than the polycarbonateprepolymer formed in the first stage reaction.
 3. A process forproducing a polycarbonate comprising (1) a first stage of melting anaromatic dihydroxy compound and a diester of carbonic acid andconducting a polycondensation reaction between the melted aromaticdihydroxy compound and diester of carbonic acid in a horizontaltwin-shaft polymerizer at a temperature from 100° to 300° C. whiledistilling off phenol, or other volatile by-products formed, to form apolycarbonate prepolymer having a viscosity-average molecular weight of5,000 to 30,000 and (2) a second stage of polycondensing thepolycarbonate prepolymer for a time of from 15 to less than 60 minutes,a temperature of from 180° to 350° C. and at a solution thickness of0.1.to 50 mm in a horizontal twin-shaft polymerizer to form apolycarbonate having a higher viscosity-average molecular weight thanthe polycarbonate prepolymer formed in the first stage.
 4. The processfor producing a polycarbonate according to claim 1, wherein the firststage reaction comprises melting the aromatic dihydroxy compound anddiester of carbonic acid in a vessel and subjecting the molten aromaticdihydroxy compound and diester of carbonic acid to polycondensation in ahorizontal twin-shaft polymerizer to give a polycarbonate prepolymerhaving a viscosity-average molecular weight of 5,000 to 30,000, whiledistilling off phenol or other compounds produced in the reaction. 5.The process for producing a polycarbonate according to claim 1, whereinthe polycarbonate prepolymer has a viscosity-average molecular weight of5,000 to 20,000.